Do we really need this printk here? Is it more of a KERN_DEBUG printk orreally just an _INFO? Either way, I suggest all UM printk's be prefixed bysomething like "um: " so it's easy to grep for them in system/console logs.

> + * XXX Deal with the lower file system changing. This could> + * be through running a tool over the top level file system to> + * make directories transparent again, or we could check the> + * mtime of the underlying directory.

Yikes, why the mention of this cache coherency issue here? If it's soimportant, then why not mention it everywhere and in the design doc? Ipersonally think trying to solve the cache-coherency in layers is too muchwork all at once: focus on basic UM functionality first. So I'd remove thiscomment from here, and add some discussion of cache coherency issues under a"Limitations" section of the design doc.